Ionization detectors are designed to provide an output signal to actuate an alarm when smoke concentration at the detector reaches a predetermined value, by detecting the reduction of ion current in the chamber caused by the presence of smoke particles. In a single chamber detector, in which the chamber electrodes are connected in series with a resistor across a power source, the change in ion current flow changes the voltage at the junction between the detector and the resistor. In a two chamber detector, having a detection chamber and a closed reference chamber connected in series across a power source, the reduction of ion current caused by smoke entering the detector chamber changes the voltage at the junction of the chambers. In a dual chamber detector, in which the reference chamber (or reference volume) is disposed within the detector chamber with a common ion source, a collector electrode is provided in the chamber between the inner and outer electrodes (sometimes forming the separation between the two chambers), and the reduction in ion current flow changes the voltage on the collector electrode. This change in voltage is utilized to actuate an alarm.
Any of the above systems must be calibrated so that the alarm output signal is provided at a definite predetermined smoke concentration. This smoke concentration provides a particular voltage at the detector.
It has been common practice to test such detectors by connecting the reference electrode to ground, or to provide a slowly increasing voltage across the smoke detecting chamber, and determining the voltage at which the alarm signal is produced. It has also been suggested that a test electrode may be provided in the smoke chamber, to be connected to ground or to a voltage varying device, to cause the alarm test signal.